body:before {
   // background: url(https://pic.imgdb.cn/item/62fca04b16f2c2beb18de0b7.jpg) center/cover no-repeat;
    content: '';
    background-repeat: no-repeat;
    background-position: center;
    opacity:0.8;
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    z-index: -1;
    -webkit-filter: grayscale(40%);
    -moz-filter: grayscale(40%);
    -ms-filter: grayscale(40%);
    -o-filter: grayscale(40%);
    filter: grayscale(40%);
    filter: gray;
}